Greenhouses and other locations for growing various plants, such as vegetables, fruits, flowers and herbs, often include artificial lighting. The electrical requirements necessary to provide the lighting can account for a significant amount of the cost of growing or producing the plants. It is desirable, then, to provide an energy efficient light for the production of plants. Many lighting technologies for the provision of artificial lighting to plants act as supplemental lighting where sunlight is the primary light source. Increasingly, artificial lights indoors provide 100 percent of the plant lighting without the benefit of the sun.
Designed for human lighting needs, existing lighting technologies for greenhouse applications are not optimal for plant growth. The response of a human eye to visible and near visible light differs from that of plants. As shown in FIG. 1, the human eye responds mostly in the visible light spectrum ranging from the green to orange wavelengths ranging from approximately 500 to 620 nanometers (nm) with a strong yellow component, with a total range from about 400 to 700 nm, with a peak around 555 nm. However, as shown in FIG. 2 and FIG. 3, plants are most responsive to the visible light spectrum corresponding to a first peak in the blue light region around 430 to 460 nm and a second peak in the red region around 640 to 660 nm.
Specifically, for the photosynthetically action radiation (PAR) range of the spectrum, chlorophyll absorption is maximized in the blue and red regions of the visible spectrum and total photosynthetic efficiency is maximized in the red region with a second local maximum in the blue region. Consequently, plant response from chlorophyll absorption is minimal in the areas of the visible spectrum that the human eye finds most responsive and maximal in the areas of the visible spectrum where the human eye is not very responsive. Therefore, lighting designed for the human eye uses energy and outputs light in areas of the visible spectrum least conducive to plant growth.
In addition to chlorophyll, plants include a variety of photoreceptors, each with an individual response as shown in FIG. 3. A broad and complex spectrum of visible light that emphasizes aspects of the blue and red spectral absorption regions is necessary to stimulate plant growth especially due to these absorption regions. For plant growth, blue light is integral for plant processes including the regulation of plant morphology, gene expression, transition to flowering, leaf expansion, phototropism, stem elongation, chloroplast positioning, gas exchange and mRNA stability. Deficiencies in blue light can negatively affect plant growth. Likewise, visible light ranging from orange to red light affects, among other things, the flowering and fruiting cycle. The most important consideration for plant growth and the design of the spectral output of lighting is the process of photosynthesis that is regulated predominantly by chlorophyll-a and chlorophyll-b. A properly broad spectrum that emphasizes the absorption regions in the blue and red spectral bands and subdues the light ranging from the green to yellow wavelengths will stimulate the plant's photoreceptors for efficient and proper growth.
Many of the lighting technologies used in grow light applications, while claiming to be beneficial to growing plants, do not provide much of their light output in the spectrum that would be most beneficial to plant growth. Additionally, much of the output light energy is in the region of the spectrum (e.g. the green to yellow wavelengths) that is either less useful or deleterious to plant growth. For example, consider FIG. 4, which shows a plot of the photon distribution versus wavelength for light output from a typical metal halide lamp used for a grow light. Metal halide lamps used for grow lighting have output peaks in the blue, green and yellow areas of the PAR spectrum. While the blue light output around 434 nm has photomorphogenic benefits including stem elongation inhibition and proper pigmentation stimulation, the large light output in the yellow and green spectra (e.g. the peaks at 545, 577 and 588 nm) and minimal light output in the red spectrum results in inefficient overall growth-stimulating light output. Furthermore, for plants to properly flower and fruit they generally require stimulation by the correct balance of red (i.e. 620 to 700 nm wavelength) and far red (i.e. 700 to 750 nm wavelength) light, which is lacking in metal halide lamps.
Referring now to FIG. 5, a plot of the photon distribution versus wavelength for light output from a typical high pressure sodium lamp used for a grow light is shown. High pressure sodium bulbs, like metal halide bulbs, generate a spectrum determined by the emission spectra of the elements inside. High pressure sodium lights output predominantly green, yellow and orange emission peaks. This emission pattern is more effective at stimulating growth than that of metal halide, but still not optimal for plant efficiency. High pressure sodium lights do not output much light below 500 nm. Consequently, high pressure sodium lights do not efficiently stimulate the photomorphogenic responses controlling plant shape and many other pigments, such as antioxidant anthocyanins and coloration pigments. Furthermore, for plants to properly flower and fruit, they generally require stimulation by the correct balance of red (i.e. 620 to 700 nm) and far red (i.e. 700 to 750 nm) light, which is lacking in high pressure sodium lights.
Referring now to FIG. 6, a plot of the photon distribution versus wavelength for light output from a typical fluorescent lamp used for a grow light is shown. Fluorescent lamp spectra bear a strong resemblance to the metal halide spectrum discussed above. Consequently, the overall effectiveness of fluorescent lamps as a grow light is similar to that of a metal halide lamp. Furthermore, fluorescent lamps generally output low photon levels relative to the requirements of plants and have short lifespans. Therefore, greenhouse environments require many units and a large amount of fixture space to achieve high light levels for plant growth and the lights are a constant source of maintenance.
Referring now to FIG. 7, a plot of the photon distribution versus wavelength for light output from a typical incandescent lamp used for a grow light is shown. Rarely used for supplemental lighting in a greenhouse environment, incandescent bulbs are inefficient at generating light in comparison to other lighting technologies. Red and far red wavelengths of light dominate the output spectrum of an incandescent bulb. Because high pressure sodium, metal halide and fluorescent lights have a very minimal far red content, incandescent bulbs remain useful for greenhouse environments for the purpose of daylength extension. However, the high proportion of far red light with respect to blue light may trigger shade avoidance responses in plants, such as stem elongation, that are often undesirable.
Recently, grow light technology may include light-emitting diodes (LED) as artificial light sources. Compared to other lighting technologies like metal halide, high pressure sodium, fluorescent and incandescent, LED lighting enables a potential reduction in energy usage. In addition, LEDs have a long life with many of the LEDs on the market rated at 50,000 hours and above. Since LEDs are a type of silicon semiconductor, LEDs output light when power is applied and do not waste energy by heating up gases or filaments that give off light like many of the other lighting technologies. Moreover, because LEDs radiate a fraction of the heat as compared to other lighting technologies, plants may be in close proximity to the LED light sources without suffering heat damage. The distance from the plants to the LED light sources may be as little as twelve inches or less.
Two key measurements when defining an LED lighting spectrum are the peak or maximum wavelength and the full-width half maximum (FWHM). The peak wavelength of the output spectrum is the wavelength of light where a global or local maximum occurs. The FWHM is the extent of the range of wavelengths on either side of the peak where the output spectrum is equal to half of the peak value. Typically, a single LED provides a narrow operable wavelength band. For wavelengths in both the blue and red regions, the FWHM is typically in the range of 15 to 25 nm FIG. 8 illustrates the spectral output of a popular brand of LED grow light with a local peak in the blue region (i.e. around 430 nm) and a local peak in the red region (i.e. around 660 nm) of the visible light spectrum where the FWHM is around 18 to 22 nm. The FWHM does not meet the wavelength range needed by most plants and, consequently, limits the ability of the LED lighting to provide the light most useful for plant growth. Overcoming this limitation by designing an output light spectrum for an LED grow light with a collection of individual LEDs with different narrow FWHMs is difficult and expensive. Additionally, LED dies other than blue and red suffer reduced electrical efficiencies, especially the green and far red dies that would broaden the LED grow light spectrum.
Most LED products designed for growing plants do not provide a light spectrum optimal for plant growth, especially when used as the primary lighting source. Current LED lighting products for grow light applications use a collection or an array of various colored LED lights to approximate a desired light spectrum. The array can include individual blue LED lights, red LED lights, white LED lights or other colors and may be difficult to assemble the collection of individual LED lights to generate the desired light spectrum. Other types of LED light applications include an array at the die level where the arrangement and mixture of red and blue dies approximate a desirable spectral light output. Regardless of how the LED array is formed, spatially diverse colored LEDs create localized areas in the plant-growing environment with varying levels of either blue or red lighting with respect to the surrounding areas. Consequently, some areas in the plant-growing environment are subject to a non-optimal spectrum of light. With an array of LEDs, mixing light outputs of individual directional LED lights creates spectrum imbalances within the light coverage area.
Further exacerbating the problem, LEDs radiate photons in a highly directional pattern. Referring now to FIG. 9, the relative angular intensity of the light output of a typical LED is shown. The LED outputs a radiation pattern where the maximum intensity occurs longitudinally from the LED (i.e. at 0 degrees) and drops off to half the intensity in approximately 45 degrees. The directionality frequently creates hot spots in the plant growth area with hot areas receiving a disproportionate or excessive amount of light compared to cooler areas, greatly affecting the plant growth. Beyond LEDs, most of the current lighting technologies for grow lights do not provide uniform lighting. That is, the lights typically have hot spots directly below the light fixture, cold spots between light fixtures, and inconsistent light levels therebetween.
As shown in FIG. 10, it is known to form a white light LED as a volumetric light emitting device 10 where a phosphor blend is molded into a three-dimensional or volumetric light conversion element 12. The volumetric light emitting device 10 includes a first reflector 14 including an aperture 16 for a light source 18 or an emitter junction, a second reflector 20 opposite the first reflector 14 for reflecting light emitted by the light source 18, and a volumetric light conversion element 12 extending between at least a portion of the first reflector 14 and at least a portion of the second reflector 20. The volumetric light conversion element 12 includes phosphor particles 22 dispersed in a resin to convert light emitted by the light source 18 from a first wavelength 24 to a second wavelength 26, the second wavelength 26 being longer than the first wavelength 24. In this way, the volumetric light emitting device 10 manages and distributes blue light and down-converted white light. Specifically, the volumetric light emitting device 10 radiates the down-converted light in a toroidal or spherical pattern. An example of a volumetric light emitting device is illustrated by Caldwell et al in US Patent Application No. 2014/0078746. Another volumetric light emitting device is disclosed in Brunt et al. U.S. Pat. No. 8,646,949 which teaches a white light scintillator.
In most LED lighting applications, the management of heat is critical to the operability of the lighting device. An LED maintaining a lower junction temperature has a longer life expectancy than when operated at a higher junction temperature. For lighting applications where LEDs are coated with phosphors, the heat generated by the LED and the heat generated by phosphor down-conversion is typically absorbed and transferred within a heat sink attached either directly to the LED or the printed circuit board (PCB) upon which the LED is mounted. In this way, conventional LED lighting applications rely on the design of the heat sink to maintain the life expectancy of the light source by maintaining a desired LED junction temperature.
In a volumetric light emitting device 10 of the type described in FIG. 10, the phosphor material that forms the volumetric light conversion element 12 is not located on or directly adjacent to the LED light source 18 and the corresponding heat sink (not shown, but either directly attached to the LED light source 18 or the PCB 19). Consequently, much of the heat generated from down-conversion does not transfer efficiently to the heat sink.
Light with a first wavelength 24 emitted from the LED light source 18 passes through a clear encapsulant 17, excites a phosphor 22 that emits light of a down-converted second wavelength 26 within the volumetric light conversion element 12 and generates heat. The heat will then generally find an exit path through the resin material in which the phosphors 22 are embedded, transfer to the sides and top of the volumetric light conversion element 12 and dissipate through convection, conduction or radiation.
A hot spot may form in the volumetric light emitting device 10. The hot spot is a localized volume in the volumetric light conversion element 12 where a concentration of down-conversion creates a localized higher temperature with respect to the overall average temperature of the volumetric light conversion element 12. A higher localized temperature may prematurely degrade the phosphor materials, the resin material, and encapsulant. Furthermore, phosphors become less efficient at higher temperatures emitting less light than at lower temperatures. The hot spot occurs, in part, because the phosphors 22 are dispersed in a homogenous fashion throughout the resin that defines the volumetric light conversion element 12. The host resin material may include many materials, such as a hardened silicone. The thermal conductivity for such a host material is typically much less than the thermal conductivity of the heat sink material, which is often aluminum or a highly thermally conductive thermoplastic. Hot spots are especially problematic if the heat created exceeds the material property specification of the resin material, the encapsulant or the phosphor materials.
Despite the homogeneous distribution of the phosphors 22 in the volumetric light conversion element 12, the distribution of the excitation of the phosphors 22 in the volumetric light emitting device 10 is not homogeneous throughout the volumetric light conversion element 12. Rather, there is a higher concentration of phosphor excitement where the light of the first wavelength 24 from the LED light source 18 initially contacts the phosphors at the interface between the encapsulant 17 and the volumetric light conversion element 12.
The maximum temperature inside the volumetric light conversion element 12 depends on several factors including the output power of the LED light source 18, the interface area between the encapsulant and the phosphors, the type of phosphors 22, the thermal conductivity of the materials, the exterior geometry, and the ambient temperature surrounding the volumetric light conversion element 12. Many of the LED light sources used today have very high power densities emitting a lot of light into a very small area.
In addition to the problems relating to a hot spot in the volumetric light conversion element, prior art volumetric light emitting devices that produce white light have been designed and are observed to shift the spectral output of a phosphor and LED. The shifted spectral output of the volumetric light emitting device enhances the spectral output of the green to yellow wavelengths and shifts the blue and red peaks towards the center of the visible spectrum.